The overlying goal of this project is to determine the developmental mechanisms by which neuromuscular junctions are initially formed. One key molecular player in the development of the neuromuscular junction is the transmembrane receptor unplugged/MuSK. Unplugged/MuSK is specifically expressed in muscle fibers and controls two aspects of neuromuscular junction formation;the clustering of "prepatterned" acetylcholine receptors (AChRs) on the muscle fiber prior to the arrival of the nerve and the guidance of the motor nerve to its proper muscle target. Later in development it also directs the formation of mature neuromuscular synapses. With the following three aims I propose to determine how unplugged/MuSK spatially coordinates clustering of "prepatterned" AChRs and growth cone guidance to the muscle center. First, I will investigate if multiple Wnt signals activate the unplugged/MuSK receptor. We have previously identified wnt11r as an unplugged/MuSK ligand, but we suspect that wnt4a is also playing a role in unplugged/MuSK activation. I will use a combination of genetic interaction and in-vivo binding experiments to examine the extent of cooperation between the two Wnts and their ability to bind unplugged/MuSK. Second, I will determine which molecular players are downstream of unplugged/MuSK and whether the signaling cascade is similar to the Wnt planar cell polarity cascade. Activation of a planar cell polarity-like pathway could establish a central zone in the muscle fiber that dictates where AChR clusters initially form and where motor axons extend when they enter the muscle territory. I will investigate the involvement of various planar cell polarity components by disrupting their function in muscle fibers and determining whether this results in unplugged- like AChR clustering and axonal guidance errors. Third, in many systems including the Drosophila wing disk, cellular polarity is initially established by asymmetric localization of various members of the planar cell polarity pathway. I propose to determine the localization of these planar cell polarity components in muscle fibers and also determine whether the absence of Wnt signaling affects this localization using in-vivo live confocal imaging techniques. PUBLIC HEALTH RELEVANCE: This research is directly relevant to the study of human diseases in that MuSK is a target of antibody- mediated autoimmune attack in the neuromuscular disease myasthenia gravis. Furthermore, mutations in components of the MuSK pathway are responsible for human congenital myasthenic syndromes.